Mobile telephones have drastically developed during past decade so that in the near future, the most developed ones will provide 2 G, 3 G and Low Power Radio Frequency (LPRF) radio communications all in the same portable device. Typically, these devices are designed to be hand held, but other form factors such as wristwatch type and wearable devices may also emerge. Common to them all, the number of antennas needed in a single device is likely to grow to two or three.
An antenna radiates electromagnetic waves with a power that is a function of its electric feed signal's power and frequency. An antenna has a resonant frequency at which it has the highest gain, which is radiation power. The highest gain not only affects the transmission efficiency but also the reception efficiency so that an antenna is also most sensitive to receive radio signals at its resonant frequency or frequencies. Hence, an antenna absorbs radio signals best at its resonant frequency.
With two or more different antennas used for different radio communications such as 3 G (Wide Band CDMA or W-CDMA) and PCS (GSM1900), for instance, the frequency bands on which these antennas operate are very close to each other or overlap, because many new radio standards share the frequency bands around 1.8–2.4 GHz region. The antennas are bound to reside close to each other if the entire apparatus housing them is small, perhaps a few centimetres in maximum dimension, and hence the coupling between the antennas is also bound to increase.
Coupling of antennas means that a portion of the radio signals transmitted by one antenna are captured by another antenna. The higher the coupling, the smaller the proportion of the transmitted radio power that actually leaves the radio device and reaches a receiver so that the transmission power will need to be boosted to ensure a reliable radio link. This naturally consumes power, causes possibly inconvenient amounts of heat dissipation and also may harm the circuitry connected to the other antenna that unintentionally captures the radio signals. It is thus necessary to ensure a sufficient level of isolation to provide satisfactory efficiency for the transmissions.
It should be appreciated that the coupling not only takes place when two different antennas are used in proximity to each other, but the mere existence of the second antenna will draw some radio power. The radio power draw is the stronger the closer the antennas are together and the closer their resonant frequencies. The isolation has often been enhanced by locating different antennas as far from each other as possible, by using different polarisations, by manually removing an unused antenna from the device for periods when the unused antenna is not needed, by placing radiation obstacles between the antennas and by disconnecting the ground or feed of unused antennas.
Due to portability requirements, the size of the radio device should be kept to a bare minimum and hence the size of printed circuit board on which the antennas typically are laid is also often too small for providing adequate isolation without dedicated measures to improve isolation.